1. Field of the Invention
This invention relates to the use of line-engraved press plates in the offset printing or lithographing of multicolor printed images having a visual halftone characteristic, and especially to the printing of said images on cylindrical objects, such as cans.
2. Description of the Prior Art
The printing or lithographing of cylindrical container or can surfaces is an established art in the container industry. The earliest procedures involved traditional offset printing in sequence on flat sheet metal or other surfaces in strip form which were subsequently cut and used to form the decorative or labeled body or side cylindrical wall of the well-known three-piece can construction. In this procedure separate colors are applied using halftone dot or grid type press plates to the flat tin plate surface, which is handled essentially in the same fashion as a paper sheet, and this printed strip is then cut to form the can cylinder to which a bottom is affixed and ultimately, after the canning process, a top lid is applied.
More recently, a can construction known as the two-piece can was developed wherein the entire can side wall and bottom were drawn from a single piece in the form of a cup and shipped to the user who would fill the can and apply a top in the normal canning process. Any printing, label or any other decoration applied to the can surface must be done after formation of the can since printing of the blank will not survive the deformation involved in the cupping or drawing procedures employed in this method. The printing techniques that have been developed for this so-called two-piece can construction are necessarily modified to provide for offset printing of multicolor images by direct application to the cylindrical surface. The methods employed include the formation of printing plates for each of several colors to be applied to the work. The traditional halftone lithograph effect by the use of printing mats prepared by the use of color separation, halftone dot or grid type screens and press plates, and overprinting have not been attainable in two-piece can printing since the problems of securing registry, proper printing pressure, angle of screen and plate, and the like to achieve the desired halftone effect attained by overprinting using halftone plates is, as a practical matter, difficult to accomplish.
The traditional grid or dot type screens used to form the halftone printing plates for multicolor printing produces a plate in which the printing surface is, in effect, a plurality of tiny raised dots or islands. The amount of color applied by each of the halftone plates was determined by the presence of and size of each of these dots, each of which, in turn, were formed photographically by traditional color separation and photoengraving techniques using appropriate filters and a halftone screen or grid. For each color, the grid and the resulting press plate is angled slightly to result in partial overprinting. The final composite multicolor shaded or toned printing product is achieved by overprinting and by optical blending, producing a visual effect of the desired tones or blends of color. Since each of the small dots formed in the press plate are fragile, the mechanical stress encountered in the application of each color to the offset surface or blanket results in smearing of the individual dots by the mechanical bending action encountered in the contact between the press plate and the surface of the offset blanket. This is especially true in the preparation of so-called flexible halftone press plates where the plate is formed by a chemical photoengraving method. In this procedure the plate is formed of a flexible, rubbery or polymeric material which has a photosensitive surface. On exposure to light, the photosensitive material is activated or hardened so that further treatment or etching, as by solvent, dissolves away the unexposed portions leaving a raised printing surface formed of a multitude of dots which corresponds to the particular color image applied to the plate. Since the base supporting the raised or printing portion is flexible, each printing operation flexes each dot and tends to smear the image and can also break the dots off.
Understandably, the quality of the image will deteriorate rapidly under these stresses, and such indeed is the experience in the can lithographing art. Likewise, the achievement of tone or color blend by traditional offset multicolor halftone techniques requires very accurate registry, careful angulation of the screens and press plates, accurate pressures, and more particularly, evenness of printing pressures. These desiderata were not attainable in the high speed printing techniques used for cylindrical objects or cans and required for economic operation in this industry. The result has been that halftone effects are not reproducible by any of the known standard multicolor offset or lithographic printing techniques which have been employed to date in the printing of two-piece cans.
Some of the prior art patents which deal with high speed can printing of two-piece metal cans include: Brigham, et al., U.S. Pat. No. 3,227,070; Brigham, U.S. Pat. Nos. 3,223,028 and 3,371,603; Freres, U.S. Pat. No. 3,385,209; and Maxin, U.S. Pat. No. 3,645,201. None of these patents, however, teach the successful use of a halftone color blend visual effect in high speed printing and all use solid color printing techniques.